Reproducing circuit



1 Dec. 5,-1933. R. s. STEWART REPRODUCING CIRCUIT Filed Nov. 10, 1928 new INVIENTOR By arfJJ'ZE-zmr UNITED STATES PATENT OFFICE Application November 10, 1928 I Serial No. 318,379

2 Claims.

This invention relates to a method and means for connecting electrical circuits for the amplification and reproduction or recording of sound waves, and more particularly relates to a method and means for coupling push-pull or modified the output energy of a push-pull amplifier to a loud speaker, telephone receiver, sound reproducer, or recorder so tion and reduce It is well known that as to minimize distorthe cost of manufacture.

in electrical sound wave amplifiers such as those used in connection with radio and telephone instruments there is always a certain amount of distortion of the speech,

music or signals. amplifiers in use having iron cores frequency currents. large proportion of amplifier, and the input, the greater employ for the The greater the the amount of distortion.

The most efiicient forms of coupling transformers amplification of audiotransformers cause a the total distortion in the amount of energy One form of amplifier which has been devised to handle a large amount of energy is the well known push-pull In the push-pull" amplifier it has hitherto been considered necessary to use a coupling transformer, choke-coils or other such coup ling device between the loud speaker and the plate circuits of the amplifier tubes. This, of course,

handled a greater amount of energy than any of the other transformers and caused a greater amount of distortion. Ampliflers, other than those of the push-pull type, which are designed to handle a great amount of energy also require an output transformer because without it the plate current of the last tube flowing in one direction'through the windings of the loud speaker would force the diaphragm or armature out of its normal position,

tortion.

causing dis- It is therefore the principal object of this invention to disclose an effective method and means the energy from the outpush-pull amfor the transference of put of a push-pull or modified plifier to the loud speaker, telephone receiver, re-

producer or recorder without the use of any of the common coupling devices.

It is a further object of this invention to provide a method and means for reducing the amount of distortion in an audio-frequency amplifying system and permitting a substantial increase in the volume of energy which may be converted.

It is a still further object of this invention to provide a method and means for increasing the electrical efficiency of an audio-frequency amplifying system by obvia ting the energy losses which would take place in a transformer or other output coupling device. I

It is a still further object of this invention to reduce the cost of manufacture of radio sets and speech amplifiers by providing a circuit in which no output coupling device between the amplifier and sound reproducer or recorder, other than a three-wire connection, is employed.

It is a still further object of this invention to provide a method and means for constructing or adapting loud speakers, telephone receivers, sound reproducers and recorders by centrally tapping their coils, and ha ing three-wire external connections instead of the customary twowire external connections.

It is a still further object of this invention to disclose a modification of the system of amplification known as push-pull amplification by connecting a loud speaker, telephone receiver, sound reproducer, or recorder, having a threewire external connection, in the place of a transformer, choke-coils, output filter or other output coupling device.

Further objects and advantages of the invention will appear from the following detailed dlsclosure thereof together with the drawing attached hereto which illustrates several forms of embodiment to which t e invention is especially adapted and, in which,

Figure 1 is a diagramof the circuit showing a type of loud speaker unit or telephone receiver, commonly used in the horn type of loud speaker, but modified by connecting a third wire to the electrical center of itswinding, connected to the output of a push-pull or modified push-pull amplifier.

Figure 2 is a diagram similar to'Figure 1 with the exception that a balanced armature unit has been substituted to operate the loud speaker or a recording device.

Figure 3 is a diagram similar to Figures 1 and 2 except that a moving coil or dynamic type of unit is used for the operation of the loud speaker or recorder.

The form of embodiment of the invention shown in Figure 1 illustrates the application of my invention to a two-pole telephone receiver, or a loud speaker unit of similar construction. Alternating or pulsating current, having the wave form of the sound or signals to be amplified is fed into the primary 1 of the input transformer and induces an alternating voltage at the outer terminals of the secondary winding 2 and 3. The secondary winding is centrally tapped at 4, and a connection is made therefrom to the filaments 5 and 5a of the two amplifier tubes 6 and 6a through wire '7, while the two outer terminals of the secondary are connected by wires 8 and 9 to grids 10 and 10a, respectively, of the two amplifier tubes. Up to this point the circuits are the same as those of the well known push-pull amplifier. But instead of employing an output transformer or choke-coils as has been done in previous forms of push-pull amplification, the windings of the loud-speaker are connected di rectly in the plate circuits of the two amplifier tubes in the following manner:

The windings of the loud speaker are tapped at the center point 12 between the two sections 13 and 14, and an electrical connection 15 is made from this center .tap to the positive terminal of the B battery 16 or other source of plate current, the negative terminal of which is connected to the tube filaments 5 and 5a. The outer terminal of the winding 13 is connected through conductor 17 to the plate 18 of one of the tubes, while the outer terminal of winding 14 is connected in like manner through conductor 19 to the plate 18a of the other tube.

The magnet 20 may be either a permanent magnet, or an electro-magnet with a direct-cur rent winding which is not shown in the diagram. The diaphragm 21 reproduces sound when actuated by variations in the magnetic flux in magnet 20. The two sections 13 and 14 of the loud speaker winding are wound in the same direction with relation to each other as if they were one continuous coil wound the entire length of the magnet 20, or, in other words, in such direction that if a current were flowing from wire 17 to Wire 19 through the two sections of the wind ing in series, the magnetic flux due to section 13 would add to that due to section 14. But when current from the battery 16 enters the windings by Wire 15 and divides, flowing in opposite directions through the two sections 13 and 14 to the plates 18 and 18a, the magnetic fiux due to the winding 13 opposes that due to the winding 14, and they neutralize each other and produce little or no efiect on diaphragm 20.

However, when speech or signal impulses are applied to the primary 1 of the input transformer they induce an alternating difference of electrical potential in the divided secondary 2 and 3, and this alternating voltage applied to grids 10 and 10a causes the current in plate circuit (a), including section 13 01" the loud speaker winding, to increase as the current flowing in plate circuit (b), including section 14 of the .loud speaker This a1ter-' winding decreases and vice versa. nately increases and decreases the strength of the magnetic field at the poles of magnet 20, thus causing vibration in diaphragm 21, which produce sound waves corresponding to the fluctuations of electrical potential on the grids l0 and 10a.

The form of construction shown herein most nearly resembles the telephone receivers now in use although the method of operation is not changed if the magnet 20 is in thevform of a straight bar rather than bent as a horseshoe. In such case, but one end of the magnet would be adjacent the diaphragm but substantially the same results would be obtained.

A slightly modified form of embodiment is shown in Figure 2 which is more advantageously adapted for a loud speaker although other reproducing devices could be used. The input current having the form of speech,.musicor signal, applied to the primary winding 30, induces an amplifier tubes 36 and 36a.

alternating voltage in the centrally tapped secondary winding 31 and 32. This voltage or difference of electrical potential is applied through wires 33 and 34 to grids 35 and 35a of the two The center tap 3'7 of the secondary winding is connected through conductor 38 to both filaments 39 and 39a. The varying difierence of potential on the grids 35 and 35a causes the current in the plate circuit (a) including plate 40, connector 41 and upper half 42 of the loud speaker winding to increase while the current in the plate circuit (12) including plate 40a, connector 43 and lower half 44 of the loud speaker winding is'decreasing, and vice versa. The windings 42 and 44 may be a continuous coil centrally tapped or two separate coils. The B battery or source of plate current 45 is connected through wire 46 to the center tap 47 of the armature winding where the current divides flowing equally thru sections 42 and 44 in opposite directions and creating a magnetomotive force in winding 42 which opposes and neutralizes the magneto-motive force in winding 44.

A permanent or electro-magnet 48 is provided adjacent the windings 42 and 44 which are in fact wound around the armature of the magnet. The armature is centrally pivoted at 49 and operates the loud speaker by direct mechanical connection.

The operation of this form is similar to that heretofore described in connection with the first form. The electrical pulsations corresponding to the sound waves cause the current in the upper half 42 of the winding to increase while that in lower half 44 decreases and vice versa. The magnetic balance-is thus disturbed causing the total magneto-motive force of the entire winding 42 and 44 to alternately assist and oppose the magnetic flux of the permanent or electro-magnet 48. This causes the vibration of the balanced armature in accordance with the wave form of the voltage applied to the grids 35 and 35a. The loud speaker may thus be operated with fewer parts and as an output transformer is not used, there will be very little distortion.

A still further modification is shown in Fig. 3 and comprises the input transformer 60 having the divided secondary 61 provided with the tubes 62 and 63 the fB battery 64 and so connected as to consist of the two balanced plate circuits (a) and (b) as heretofore described.

ihe dynamic speaker magnet 66, which is of annular shape, carries the floating coil 65 centrally tapped to provide the equal sections 67 and '68. The armature is mechanically connected with the loud speaker 69.

The operation of this form is the same as that of the preceding forms. The fluctuations of electrical potential having the wave form of sound waves impressed on the grids cause variations in the intensity of the current in the two halves of the winding of the movable coil 65. The current in one section increases while the opposing current in the other section decreases, causing are not intended to be complete and that the location of the A and C batteries as well as any resistances or reactances necessary to compensate and balance the (a) and (b) circuits are now so well known and as they do not form a part of this invention, and would not aid the clarity of this disclosure, they have been left out.

This invention will also reproduce with the least distortion and with the greatest electrical eficiency, small variations in electrical energy other than those due to sound waves, which may be impressed directly on the input transformer. Further, if other amplifying devices were used in lieu of vacuum tubes, they would not vary the operation of this invention nor affect the principal advantages thereof, which are reduced equipment, increased efficiency, reduced distortion and reduced cost.

It will be understood that separate anode batteries may be used for the two tubes if the two sections of the loud speaker windings are not directly connected together, but each connected to the positive terminal of its anode battery or source of plate curren It will also be understood that the two sections of the loud speaker winding need not be placed end to end, but one section may be wound over the other, or they may be in the form of alternate layers or spirals or the wires of the two sections may be placed tightly together and wound as a double conductor throughout their length for the purpose of securing more effective magnetic neutralization.

While I have shown these various forms of em bodiment of my invention, I am aware that various modifications may be made thereto and I therefore desire protection on the broad scope and spirit of the disclosure taken in conjunction with the attached claims.

' I claim:

1. In a system for the electrical amplification and reproduction of speech and neutralization of direct current components, means to divide the speech currents into two components, means to amplify each component separately, a divided coil loudspeaker, and means to feed said components from the amplifying means directly into the two halves of the divided coil loudspeaker in such phase that the pulsating or speech components of the current in the two halves of the divided coil exert a cumulative magnetic force on the diaphragm of said loudspeaker, said direct current components being magnetically opposed to exert little or no magnetic force on the diaphragm.

2. The system for amplifying and reproducing sound, having in its final'stage two tubes with their inputs so connected to .the divided secondary of an input transformer as to cause the speech current in the plate circuit of one tube to increase while the speech current in the plate circuit of the other tube is decreasing, and vice versa, and having a sound reproducing unit whose windings are tapped at the center and the center tap connected to the positive terminal of the source of plate current and the two outer terminals of said winding connected one to each of the plates of the two amplifier tubes, said sound reproducer winding providing efiective neutralization of the magnetic flux created by equal currents flowing in opposite directions in the two sections of said winding while speech cur- 

